1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic technology, and in particular to an image forming apparatus which is provided with a light generation device such as a charge removing member for radiating light having a specific wavelength to an organic photosensitive film located on a surface of a photosensitive drum and forms an image by charging and exposing the photosensitive drum to light.
2. Description of the Related Art
Conventionally, image forming apparatuses using electrophotographic technologies have been actively developed for use as, for example, electrostatic copiers or printers.
Briefly referring to FIG. 3, a conventional image forming apparatus 1 using an electrophotographic technology will be described. An image forming apparatus 1 includes a rotatable photosensitive drum 3 having a photosensitive film 2 located on a surface thereof, a main charger 4 for uniformly supplying the photosensitive film 2 with a prescribed level of electric charge, an optical device 5 for exposing the photosensitive film 2 to light and forming an electrostatic latent image on the photosensitive film 2, a developing device 6 for developing the electrostatic latent image formed on the photosensitive film 2 into a toner image, a transfer device 8 for transferring the toner image on the photosensitive film 2 onto a recording paper sheet 7, a cleaning device 9 provided with a cleaning blade for removing the residual toner on the photosensitive film 2, and a charge removing device 10 for removing the residual charge on the photosensitive film 2 and thus setting the surface potential of the photosensitive film 2 at a prescribed uniform level.
In the image forming apparatus 1 having the above-described structure, an image is formed in the following manner.
First, the main charger 4 uniformly supplies the photosensitive film 2 with a prescribed level of electric charge. Next, light is radiated to the photosensitive film 2 by the optical device 5, thereby forming an electrostatic latent image on the photosensitive film 2. Toner is supplied to the photosensitive film 2 by the developing device 6, thereby developing the electrostatic latent image into a toner image. The toner image on the photosensitive film 2 is transferred to the recording paper sheet 7 by the transfer device 8. After the transference, the residual toner on the photosensitive film 2 is removed by the cleaning device 9. Light is radiated on the photosensitive film 2 by the charge removing device 10, thereby removing the residual charge on the photosensitive film 2. Thus, the surface potential of the photosensitive film 2 is uniformly set at a prescribed level. Thereafter, the photosensitive film 2 is charged again by the main charger 4. Such a process is repeated in accordance with the rotation of the photosensitive drum 3.
The photosensitive film 2 is formed of an inorganic or an organic material. Usable inorganic materials include, for example, Se-type materials and amorphous Si-type materials.
Recently, more and more photosensitive films are formed of an organic material due to higher safety and easier processibility thereof. Organic photosensitive bodies formed of an organic photosensitive material are classified into multiple-layer organic photosensitive bodies and single-layer organic photosensitive bodies.
A multiple-layer photosensitive body includes a charge generation layer and a charge carrying layer. The charge carrying layer contains a charge carrying material. The charge carrying material may be a hole carrying material or an electron carrying material. There are various hole carrying materials which have a satisfactory carrying ability, whereas no electron carrying material having a satisfactory carrying ability has been developed. Accordingly, multiple-layer organic photosensitive bodies are mostly of a type to be negatively charged. However, when such a type of photosensitive body is charged using a charger for performing corona discharge, a large amount of ozone is generated. In order to protect the human body and the environment, an additional measure to deal with ozone is needed.
In an attempt to solve the above-described problem, single-layer organic photosensitive bodies have been developed. A single-layer organic photosensitive body includes a charge carrying medium containing a charge generation material diffused therein. By using a charge carrying material having a satisfactory hole carrying ability as the charge carrying medium, a single-layer photosensitive body of a type to be positively charged can be easily formed.
Compared to a multiple-layer organic photosensitive body, a single-layer organic photosensitive body is easier to produce and thus is preferable. However, a single-layer photosensitive film formed of a single-layer organic photosensitive body retains a generally high level of charge even after charge removal. If the photosensitive film is positively charged in the state of having residual electrons therein, such residual electrons move to a surface of the photosensitive film due to the surface potential obtained by charging and thus reduces the surface potential.
FIG. 4 is a graph illustrating the surface potential and the residual potential with respect to repeated rotation of a photosensitive drum formed of a conventional organic photosensitive material. In the case that an image is formed by the repeated rotation of the photosensitive drum, the level of surface potential reduces significantly and the level of residual potential increases significantly, causing non-uniformity in the image density. Further, residual photocarriers in the organic photosensitive film which causes such a high level of residual potential undesirably reduces the surface potential obtained by charging. Moreover, the organic photosensitive film can be initially charged sufficiently but wears out while being used repeatedly. Accordingly, the trap site is increased and thus the organic photosensitive film becomes more difficult to be charged.